A Coriolis flowmeter is a mass flowmeter that supports both ends of a measuring tube through which a fluid to be measured flows, and detects a phase difference signal proportional to a mass flow rate at symmetric positions between both-end supported portions and a center portion of the thus supported measuring tube at a time when the center portion of the supported measuring tube is alternately driven in a direction perpendicular to a line of support. The measuring tube is formed of a metal material having a Young's modulus that changes with a temperature. Hence, in order to maintain high measurement accuracy, it is necessary to measure the temperature of the measuring tube, and to compensate for the change of the Young's modulus, which follows a change of the temperature.
A description is made below of the temperature measurement using the Coriolis flowmeter by way of a conventional example thereof (for example, see Japanese Patent No. 3105253). As illustrated in FIG. 6, in the conventional temperature measurement, a bridge voltage is input to a subtraction circuit by using a resistor bridge, and a voltage output therefrom is input to a V/F converter, is converted into a frequency, and is arithmetically converted into resistance in a CPU, whereby the temperature is determined. Reference numeral 1 of FIG. 6 denotes a platinum temperature measuring resistor (PT100: 100Ω) as a resistor-type temperature sensor. Further, reference numeral 2 denotes a resistor bridge unit; 3, electric wires having line-to-line resistors (RC); 4, a voltage reference (VCC); 5, a reference resistor (Rref: 100Ω); 6, the V/F converter; and 7, the CPU.
In the V/F converter 6, for example, such an output of the frequency therefrom is represented by the following expression.
            [              Expression        ⁢                                  ⁢        1            ]        ⁢                                                          F            out                    =                                    V              ⁢                                                          ⁢                              3                ·                R                            ⁢                                                          ⁢              12                                                      2.09                ·                R                            ⁢                                                          ⁢                              8                ·                R                            ⁢                                                          ⁢                              10                ·                C                            ⁢                                                          ⁢              1                                                                                
As understood from the above-mentioned expression, in the V/F converter 6, the frequency output is decided by ratios among resistors and a capacitor, and accordingly, temperature characteristics of the resistors and the capacitor are extremely important. In particular, the capacitor is inferior to the resistors in temperature characteristics, and accordingly, the frequency output is affected by ambient temperature of a circuit every time when the ambient temperature thereof is changed. Therefore, the V/F converter 6 has a problem of being sometimes incapable of performing accurate temperature measurement.
When the temperature is attempted to be determined in the above-mentioned configuration while defining the bridge voltage of the platinum temperature measuring resistor 1 as V1, the bridge voltage of the reference resistor 5 as V2, and the output voltage from the resistor bridge unit 2 as V3, V1 to V3 are represented by the following expressions.
            [              Expression        ⁢                                  ⁢        2            ]        ⁢                                    ⁢                  V        ⁢                                  ⁢        1            =                                                  PT              ⁢                                                          ⁢              100                        +                          R              ⁢                                                          ⁢              C                                                          PT              ⁢                                                          ⁢              100                        +                          R              ⁢                                                          ⁢              C                        +                          R              ⁢                                                          ⁢              1                                      ⁢                  VCC          ⁢                                          [                      Expression            ⁢                                                  ⁢            3                    ]                                        ⁢                  V        ⁢                                  ⁢        2            =                                                                          ⁢                          100              +                              R                ⁢                                                                  ⁢                C                                                                                                ⁢                          100              +                              R                ⁢                                                                  ⁢                C                            +                              R                ⁢                                                                  ⁢                4                                                    ⁢                  VCC          ⁢                                          [                      Expression            ⁢                                                  ⁢            4                    ]                                        ⁢                  V        ⁢                                  ⁢        3            =                                                  R              ⁢                                                          ⁢              5                                      R              ⁢                                                          ⁢              2                                ⁢          V          ⁢                                          ⁢          1                -                                            R              ⁢                                                          ⁢              4                                      R              ⁢                                                          ⁢              3                                ⁢          V          ⁢                                          ⁢          2                    
As understood from the above-mentioned expressions, the output voltage V3 from the resistor bridge unit 2 is decided by ratios among at least six resistors. A problem here is that temperature characteristics of the six resistors simultaneously affect the output voltage V3 though it is understood that the temperature characteristics of each of the resistors are extremely good. This is an extremely serious problem in performing the accurate temperature measurement.